Project summary: Despite advances in melanoma treatment, only 33% of patients with advanced disease respond to the most effective therapy and mean survival is only 23 months. Millions of people with red hair, or light skin pigmentation have an especially high melanoma risk. Although strategies to decrease this risk are lacking, our recent discoveries suggest that melanoma incidence may be diminished by pharmacologically activating the G-Protein Estrogen Receptor (GPER), a protein on melanocytes with activity completely distinct from the classic estrogen receptor (ER?/?). Although there are no approved drugs that target GPER, GPER is activated in melanocytes by a selective synthetic compound (G-1) that does not have any classic estrogen activity. We recently determined that pharmacologic GPER activation in vivo inhibits established melanomas, largely by inducing terminal differentiation in cancer cells. Our preliminary studies now suggest that G-1 mediated GPER activation in melanocytes induces long-term epigenetic changes that prevent future melanoma, while allowing skin melanocytes to continue to function normally. In Aim I we will use primary human melanocytes to determine the specific histone modifying enzymes required for inducing epigenetic transcriptional memory that maintains a heightened state of cellular differentiation after transient GPER activation, and test whether HDAC inhibition potentiates the differentiation effects of the GPER agonist. In Aim II we will validate preliminary results suggesting that GPER signaling induces pathways that promote DNA repair, and thereby minimizes the accumulation of DNA mutations after ultraviolet (UV) exposure. We will determine the mechanism(s) downstream of GPER that mediate the improved DNA damage response, which may help highlight additional therapeutic targets. In Aim III we will use both human and mouse melanoma models to directly test whether G-1 activation of GPER promotes DNA repair after UV exposure in vivo, and inhibits melanoma development.